The present invention relates to boreholes which are drilled for exploration or for exploiting underground wealth. The invention more particularly relates to evaluating with ultrasonic tools the quality of the cement which surrounds a casing in a borehole, the cement being inserted in the borehole to hold the casing and to perform the function of hydraulic sealing.
Ultrasonic tools for investigating the environment surrounding the casing are known in the art. For example, the CBL tool (Cement Bond Log), uses an ultrasonic emitter and an ultrasonic receiver which are spaced apart along the casing. The CBL tool operates according to the general principle that if the receiver rapidly receives a strong echo, then the casing is vibrating and is not being supported by cement. On the other hand, if the strong echoes arrive late at the receiver, or if no strong echoes arrive at all, it may be supposed that the casing is properly held by the cement which surrounds it. Each measurement performed by the CBL tool is applicable to a length of casing substantially equal to the distance between the tool's ultrasonic emitter and receiver.
Another tool which has recently appeared for testing the environment surrounding a casing is known as a CET (a trademark of Schlumberger) tool (Cement Evaluation Tool). In contrast to the CBL tool, the CET tests the casing in radial sectors. To accomplish the same, the CET tool generally comprises eight ultrasonic emitter/receiver transducers which are spaced both angularly and axially (e.g. along a helix) relative to the casing. The CET preferably additionally comprises a reference transducer together with a reflector which is located at a fixed distance relative thereto. The reference transducer is arranged to provide the in situ speed of ultrasound inside the casing.
With the CET tool arranged as described, it is thus possible to perform eight acoustic diameter measurements and to subsequently determine the degree of any ovalization of the casing as well as any off-centering of the sonde carrying the tool relative to the casing axis. Moreover and more importantly, the CET tool makes it possible to evaluate the quality of the cement to some degree in several angular directions in each cross section of the casing. Thus, the CET tool is good at determining over the various angular directions scanned by the transducers whether the casing is free to vibrate in a thickness resonance mode, i.e. the casing is surrounded by gas, or whether it is well held by the cement. In contrast, situations intermediate to the casing being well held ("cement domain") and the casing being free to vibrate ("gas domain") are more difficult to analyze.
Presently, it is the practice in such intermediate situations (the "intermediate domain") to deduce measurements of the cement's Resistance to compression in each angular direction scanned by the transducers and to characterize the cement therefrom in terms of its mechanical strength. However, the inventors have observed that the compression resistance is an imperfect representation of the quality of the hydraulic seal which is sought around the casing between the various underground layers through which the borehole passes. In particular, it can happen that cement whose compression resistance appears to be satisfactory is in fact permeable to gas, in particular via communicating pores.